A data processing facility typically includes one or more host devices to which are attached storage devices, either directly or through a network. As a customer's business grows, the need for increased storage also grows. However, installing additional high speed storage devices, such as DASD (direct access storage device), can become expensive. Moreover, not all of a customer's storage may need to be of the expensive, high speed variety. Rather, such storage may be allocated to data which is to be accessed frequently while less frequently accessed data may be stored on slower and less expensive storage devices, such as tape cartridges. Storage with multiple levels is said to be configured as hierarchal storage.
The operating system of the host may include a hierarchal storage manager. The OS/390® or z/OS® operating system which operates on an IBM® S/390® server may include a Data Facility Storage Management Subsystem (DFSMS™) which automates storage management policies and procedures to move data up and down a storage hierarchy. It will be appreciated that other vendors may provide other products which also manage hierarchal storage space. And, while the present invention will be described in the context of DFSMS from IBM, the invention is not limited to being incorporated into that particular product.
One component of DFSMS is the hierarchal storage manager (DFSMShsm™, also referred to herein as “HSM”) which oversees backup, recovery, migration and space management operations. One of the functions provided by the space management function of DFSMShsm is “interval migration” which directs the periodic migration of data sets to locations where they are stored less expensively or in a form which saves space. HSM interval migration automatically manages volumes in DASD storage space (also known as Level 0) based upon user-specified constructs. Volumes storing data sets which have common storage requirements, as defined by the customer, may be grouped together in “storage groups”. One of the parameters which the customer may select for a volume is a “high threshold”. At predetermined intervals, HSM examines DASD volumes in alphabetical order and determines whether the space used by the data sets stored to a volume exceed the high threshold. If so, data sets in that volume are eligible for migration from Level 0 to Level 1 (such as DASD/tape media). Migration will then be performed on all such volumes, beginning with the largest data set in each volume and continuing with progressively smaller data sets until another user-defined parameter, the volume's “low threshold”, is reached.
Another function of DFSMS is the allocation of space for new data sets. Typically, space for a new data set will be allocated to the volume which has the most amount of free space. Frequently, however, the volume to which space is allocated is the same volume from which data sets are being migrated. Such a conflict causes contention for common resources, including volume table of contents (VTOCs), catalog entries and the like, resulting in a slowdown in data migration and/or customer workload.
Consequently, a need exists for reduced contention between data migration and space allocation as well as for increased migration speed.